RUBY-713 Use createIndexes command when available

Use the createIndexes command when creating an index instead of inserting into system.indexes. If the command isn't available, fall back to inserting into system.indexes.

Note: I removed a line that handled an error case

    if selector[:dropDups] && e.message =~ /^11000/

because https://jira.mongodb.org/browse/SERVER-2054 was resolved in 2.0, which is eol'ed.